SPECIAL REPORT 97-1

Frost Shielding Protection of a

Water Line, Berlin, New Hampshire

Barry A. Coutermarsh January 1997
Abstract: The standard practice of burying water

and sewer lines beneath the frost line in cold regions

can be expensive when ledge or other difficult material is within the burial depth. If the pipeline can be

buried at a shallower depth and still be protected

from freezing, a significant savings in excavation

costs can be realized. A finite element (FE) program

was developed to predict frost penetration depth

around buried utility pipelines. The program was

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used to design and assess the feasibility of various

insulation configurations around a water line buried

within the frost-susceptible depth in Berlin, New

Hampshire. Extensive temperature monitoring was

performed to evaluate both the insulation design

and the prediction accuracy of the FE program. The

first-year results are very promising, showing good

accuracy between the FE results and actual temperatures.
Special Report 97-1

Frost Shielding Protection of a

Water Line, Berlin, New Hampshire

Barry A. Coutermarsh January 1997

Prepared for

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared by Barry A. Coutermarsh, Research Civil Engineer,

Applied Research Branch, Research and Engineering Directorate, U.S. Army Cold

Regions Research and Engineering Laboratory (CRREL), Hanover, New Hampshire.

Funding for this work was provided under the Construction Productivity Advancement Research (CPAR) program. The report was technically reviewed by Dr.

Gary E. Phetteplace of CRREL and Dr. John Budinscak of U.C. Industries, Inc.

The contents of this report are not to be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.

ii
CONTENTS

Page

Preface ................................................................................................................... ii

Introduction .......................................................................................................... 1

Background ...................................................................................................... 1

Numerical model ................................................................................................. 2

Second Avenue .................................................................................................... 4

Labossiere Street .................................................................................................. 6

Initial design .................................................................................................... 6

Final design ...................................................................................................... 6

Construction ......................................................................................................... 7

General physical layout ................................................................................. 7

Pipe construction ............................................................................................ 7

Instrumentation layout ....................................................................................... 10

Preliminary data .................................................................................................. 11

Air temperatures and frost penetration....................................................... 11

Pipe temperatures ........................................................................................... 12

Shield temperatures ........................................................................................ 12

Numerical model assessment ............................................................................ 13

Preliminary conclusions and recommendations ............................................. 15

Literature cited ..................................................................................................... 15

Abstract ................................................................................................................. 16

ILLUSTRATIONS

Figure

1. Example of a finite element mesh ......................................................... 2

2. Average air temperatures from 1972 used to represent FE

surface temperature ......................................................................... 3

3. Berlin, New Hampshire, filter plant water temperatures .................. 4

4. Physical layout of FE-modeled domain ............................................... 4

5. FE output showing isothermal progression ........................................ 5

6. Storm drain location, immersion thermocouple, and the start of

the insulated pipe section on Labossiere Street ........................... 8

7. Instrumentation layout along the pipeline .......................................... 8

8. Jigs made from 2- × 4-in. lumber to hold the insulation during

installation ......................................................................................... 9

9. Staggered insulation board joints .......................................................... 9

10. Insulated service tap entering the shield ............................................. 9

11. Residential sewer line crossing through the shield ............................ 10

iii
Figure Page

12. Sewer line crossing with thermocouple layout ................................... 13

13. Thermocouple layout at the test site ..................................................... 13

14. Air temperature vs. FE surface temperature ....................................... 12

15. Control string vertical temperature profiles ........................................ 12

16. Comparison of lower and middle immersion thermocouples

and filter plant temperatures .......................................................... 12

17. Immersion temperature vs. ground temperature ............................... 13

18. Comparison of lower immersion thermocouple, underdrain,

and earth temperatures ........................................................................... 13

19. Inside vs. outside shield temperatures ................................................. 13

20. Pipe vs. ground temperature ................................................................. 13

21. FE surface temperature vs. actual surface temperatures,

Laboissiere Street .............................................................................. 14

22. FE surface vs. actual temperatures........................................................ 14

TABLES

Table

1. Numerical model material data ............................................................... 5

2. Insulation data ............................................................................................ 7

iv
INTRODUCTION

In areas that experience freezing temperatures

for any appreciable time, protecting water lines

from freezing is a major consideration in the design and construction of water systems. The normal procedure is to bury the systems below the

anticipated frost penetration depth. The presence

of ledge above this depth can greatly increase the

cost of burying the system. This is not only because of the expense of blasting and removing the

rock but also because frost penetration is deeper

in areas of ledge, requiring an even deeper pipe

burial depth to protect against freezing.

Frost shielding is the practice of protecting the

water line from freezing by adding extruded polystyrene insulation to retard the heat loss from the

water within the line. It has the potential of reducing the burial depth of the system from what

would normally be necessary. Balanced against

the increased cost of the extruded insulation is

the decreased depth required for safe burial of

the pipe and thus savings in both the volume and

cost of removed material.

The major impediments to routinely insulating

lines are the lack of performance data and design

guidance for the shield configuration and insulation thickness. A finite element (FE) computer

program at CRREL can model two-dimensional

frost penetration into the ground and describe

both numerically and visually the temperature

regime expected in and around any potential

shield design. This FE program allows the designer to model several different insulation configurations and perform “what-if” types of calculations against expected temperature conditions.

This report illustrates its use in the design of

an insulation shield for an 8-in. (20-cm)-diam.

water line in Berlin, New Hampshire, in an area

where ledge is present to the surface. It details the

construction of the shield, methods used during

construction to handle the 2- × 8-ft (0.6- × 2.4-m)

extruded polystyrene insulation boards that make

up the shield, and the temperature sensor layout

we used to monitor the shield’s performance. The

project will be used to assess the performance of

both the FE program and the insulation shield

under in-situ conditions.

Background

This frost-shielding project was developed under the Corps of Engineers, Civil Works, Construction Productivity Advancement Research

(CPAR) program. Under the program, the City of

Berlin, New Hampshire, Water Works and U.C.

Industries, Inc., a manufacturer of extruded polystyrene insulation, are partners with CRREL in

demonstrating the concept of insulating water

lines to protect them from freezing. The project

has essentially two components. The first is to

develop a shield design using a CRREL-developed FE program. The second is to construct and

assess the performance of an in-situ shield based

upon the numerical design. More information

about the finite element program is available in

Coutermarsh and Phetteplace (1991a, 1991b).

The practice of frost-shielding utility lines has

been used most notably in Norway, where Per

Gunderson has been instrumental in its development. He has developed nomographs to estimate

the shield configuration and thickness using certain environmental parameters. The Norwegians

have also pioneered collocating sewer lines within

the same shield as water lines. The warmer sewer

lines supply heat to the shield, which helps to

keep inside-shield temperatures above freezing

(Gunderson 1975, 1989).

It was the success of Gunderson’s work that

encouraged us to further investigate the concept

of frost shielding in this country. We felt, howFrost Shielding Protection of a

Water Line, Berlin, New Hampshire

BARRY A. COUTERMARSH
ever, that the nomographs were of limited use in

conditions where several different soil materials

were present, the geometry of the pipeline was

nonstandard, or the ambient temperature profiles

were erratic. With the advent of more powerful

personal computers, we felt that an FE program

would offer the designer more flexibility in choosing shield design based upon the conditions present.

The city of Berlin, New Hampshire, situated in

the White Mountain range of northern New

Hampshire, has a population of about 12,000. The

town’s geography is mountainous with large

areas of shallow ledge throughout. The Berlin

water department approached CRREL to see if

we could offer any advice on the freezing problems they were experiencing with their water distribution system.

The water in the distribution system is from

two sources: the Ammonoosuc Filter Plant and

the Androscoggin Treatment Plant. The Ammonoosuc plant gets its water off the bottom of a

reservoir, and the Androscoggin plant is supplied

from the Androscoggin River. The Ammonoosuc

plant only filters and chlorinates the water, but

the Androscoggin plant provides a full range of

treatment. During the winter, the water leaving

the Ammonoosuc plant tends to be roughly 2°C

warmer than the water leaving the Androscoggin

plant.

The distribution system itself is old, with many

of the lines made up of small galvanized 2-in. to

6-in. (5.1 to 15.2-cm)-diam. pipes. A large portion

of these pipes are buried in or on top of ledge,

tend to be very shallow, and are prone to freezing. The frost penetration in this area of New

Hampshire can be 1.5 m (5 ft) or more. The problem is so acute that from January through March

the water users on lines prone to freezing are

asked to run their water continuously. They are

given an abatement on their water fees by being

billed on the average of the previous three months’

usage rather than on their actual usage during

the freezing period. This causes a situation where,

during the winter months, water is being run

through the system just to prevent freezing. The

cost of this water is in effect subsidized by all the

users. The excess water is discarded into the sewer

system, where it puts a greater load on the sewer

treatment system and therefore increases treatment cost.

The city is in the process of upgrading its water system to correct this situation. The presence

of shallow ledge, however, makes this an extremely costly procedure. Ledge excavation is relatively expensive because of the need to blast to

break up the material for removal. The situation

is exacerbated because frost penetrates much

deeper in ledge than it does in granular soils. This

results from two factors: first, the ledge has a

higher thermal conductivity than soil, and second, because it has no moisture content, it has no

latent heat (the energy used when changing the

phase of a material, e.g., changing water to ice). It

can be seen that an effective shielding method

that would allow a shallower burial depth than

would otherwise be necessary would have the

potential to save a substantial amount of money

and resources. U.C. Industries, Inc. expressed an

interest in the concept of frost-shielding and became a partner with the water department to provide guidance and insulation for the project as

well as to be the technology transfer point of contact to private industry.

This report describes the numerical design and

construction of a 20.3-cm (8-in.) shielded water

line in Berlin, New Hampshire. The ongoing

project will evaluate its performance and adjust

the design procedure accordingly as data are

evaluated.

NUMERICAL MODEL

The physical dimensions of the pipeline and

surrounding material are used to construct an FE

mesh (Fig. 1). Only half of the physical configuration needs to be modeled, since the other half will

be identical to the modeled half. A mesh is therefore constructed that represents one side of a ver0

–0.5

–1.0

–1.5

–2.0

–2.5

Figure 1. Example of a finite element mesh.

2
tical line of symmetry through the insulation and

pipe. This mesh consists of a series of triangular

elements arranged in zones that accurately represent the physical configuration of the material

being studied. The density of the element spacing

depends upon the temperature gradients expected

in any zone. Areas of steep gradients (high temperature change) should have elements that are

spaced closely together. Thus, in Figure 1 there

are areas that have very closely spaced elements

representing the insulation boards. These are the

darker vertical and horizontal bands evident in

the figure. The insulation boards are placed in an

inverted U immediately around the pipe, which

is the darker half-round object at the lefthand

side of the mesh. The dark bands extend beyond

the boundaries of the insulation boards because it

is easier to generate them as a continuous band

throughout the mesh and then just designate the

board itself in the material file by their coordinates. For example, the top horizontal board extends from the lefthand side of the mesh to the

righthand edge of the vertical band. The vertical

board extends from the bottom of the horizontal

band to just below the bottom of the pipe. Each of

the other materials present in the physical configuration is designated by the appropriate coordinates of the zone of elements in the mesh. The

program uses this information, along with the

thermal conductivity, density, specific heat, latent heat, and phase-change temperature of the

components to determine heat transfer throughout the region under study. More information on

finite element (FE) modeling can be found in textbooks on the subject (Segerlind 1984).

Once the physical configuration is determined,

all the boundary conditions that will affect the

mesh temperatures have to be determined for a

full year’s cycle. In our case, the surface

temperature, water temperature, and

geothermal heat flux (heat generated

within the core of the earth that slowly

filters up to the surface) were the boundary conditions that were needed for our

problem. Along the vertical line of symmetry, the boundary condition is adiabatic (zero heat flux).

The surface temperatures used in the

model were determined from monthly

extreme air temperature records for the

Berlin area from 1926 to 1992. From the

records, 1972 appeared to be the coldest year, and the average of the coldest

and warmest temperatures within each

month were used as our surface temperatures in

the model.

Actual surface temperature is usually different

from air temperature and can be determined by

the use of so-called “n-factors.” These factors relate the effect that surface material has upon the

surface temperature (Lunardini 1981). By multiplying the air temperature by the appropriate

material n-factor, the surface temperature can be

determined. A good illustration of the surface temperature difference is the hot temperatures found

in summer on a black asphalt surface, which will

normally be several degrees hotter than the air

temperature.

Using air temperature as our mesh surface

boundary temperature gives us, on average, a

somewhat colder surface than the actual would

be. This was desirable from a conservative point

of view so our shield design would be somewhat

overdesigned. Figure 2 is a graph of the air temperatures determined from our records. The time

scale is in months and represents a full year starting in January.

Water temperature was more difficult to determine. We obtained monthly treatment and filter

plant outflow water temperatures for 1992 and

part of 1993 and used these records to estimate

the water temperature in the pipe. There can be a

significant error in these temperatures because

the water temperature will change as it travels

throughout the distribution system. How it

changes is not known. For instance, in the winter,

the water from the plants might cool significantly

as it travels through pipes that are buried in

ground that is below freezing. Conversely, since

a large portion of Berlin’s water is from a surface

source, it may in fact be cooler than the ground

for a large part of the season. The preliminary

20

10

 0

–10 Temperature (°C)

Jan Jun Jul Feb Mar May Apr Aug Oct Sep Dec Nov

Figure 2. Average air temperatures from 1972 used to represent

FE surface temperature.

3
data, which we will show later, indicates that

early in this year the water at the filter plant was

colder than the ground for portions of the year.

Figure 3 shows the water temperatures we used

in our simulation.

In our numerical simulations we model to a

depth of 10 m (32.8 ft), which can reasonably be

assumed to represent the depth of zero annual

temperature amplitude (Phukan 1985). This is the

depth at which the effect of the changing surface

temperature is not noticeable. At this bottom

boundary we apply an average value of the geothermal heat flux of 0.063 W/m2 (Lunardini 1981)

moving upward into the mesh.

SECOND AVENUE

Figure 4 is an example of the basic FE configuration used to model what we initially expected

the soil and pipe configuration to look like at our

original test site on Second Avenue in

Berlin. The pipe is 20.3-cm (8-in.) I.D.

ductile iron set 1.5 m (5 ft) deep. Material 6 is the pipe and water, material 4

represents the backfill area around the

pipe, material 7 is the extruded polystyrene insulation boards, and materials 2 and 11 are the surrounding earth

and ledge material, respectively. The

specific dimensions and materials associated with each zone might change, depending upon what the designer wishes

to experiment with. For instance, in our

modeling we varied the pipe depth, insulation board thickness, and the width of the top

insulation board to see the effect these changes

would have upon the temperature profiles.

We thought that the pipe on Second Avenue

could be installed about 1.5 m (5 ft) deep, but

ledge was possible anywhere from 1.4 m to 2 m

(4.5 ft to 6.5 ft). In Figure 4, material 11 was ledge

and material 2 was a “silty sand” commonly found

in the area. The backfill material 4 is a sand. As

mentioned above, we varied the extruded polystyrene insulation (material 7) dimensions during

the modeling process to investigate the effectiveness of different shield thicknesses and placement. Table 1 lists the materials and their properties.

During the numerical simulations, we defined

a failure as the 0°C isotherm touching the pipe.

This is a conservative approach, since water will

remain at 0°C for some time before it freezes due

to its latent heat. For the initial runs we did not

apply any water temperature boundary condition to the pipe. This would be similar to a

pipe with no flowing water. Commonly, water flowing in the pipe brings heat into the

insulation shield, so modeling without water flow is usually a more conservative procedure.

The output from a numerical simulation

is presented in graphical form by either

graphs or contour plots. By watching how

the 0°C isotherm progresses with time, the

effectiveness of the insulation shield can be

determined. Figure 5 shows a time series progression of five isotherms around and into

a shield. Notice how the isotherms wrap

around the shield and start to intrude not

only at the top but also at the lower right

corner. This is due to the higher-conductivity ledge at the bottom of the shield. This

contour type of output is extremely helpful

16

12

8

4

 

0

Temperature (°C)

Jan Jun Jul Feb Mar May Apr Aug Oct Sep Dec Nov

20

Figure 3. Berlin, New Hampshire, filter plant water temperatures.

0 0.5 1.0 1.5 2.0

0

–0.5

–1.0

–1.5

–2.0

y

x

Material 4 Material 2

Material 6 Material 11

Material 7

Figure 4. Physical layout of FE-modeled domain.

4
Table 1. Numerical model material data.

Conductivity Density Specific heat Latent heat

Material and state (J/hr m °C) (kg/m3) (J/kg °C) (J/m3)

Material 4

Sand

Frozen 5616 1836 784.8

Unfrozen 6228 1836 940.32 45.3 × 106

Material 6

Water

Frozen 7984.8 917.0 2096.2

Unfrozen 2170.8 998.2 4183.92 334 × 106

Material 7

Extruded polystyrene

104.04 28.84 1214.23

Material 11

Rock

9242 2700 879

0 0.5 1.0 1.5 2.0

0

–0.5

–1.0

–1.5

–2.0

y

x

Temperature –2°C

–1

0

1

2

0 0.5 1.0 1.5 2.0

0

–0.5

–1.0

–1.5

–2.0

y

x

Temperature –2°C

–1

0

1

0 0.5 1.0 1.5 2.0

0

–0.5

–1.0

–1.5

–2.0

y

x

–1

Temperature –2°C

0

a. Mid-December.

b. Mid-February. c. Early April.

Figure 5. FE output showing isothermal progression.

The time steps are 12 h apart.

5
in assessing the shield design. A large amount of

information can be displayed in a format that is

easily interpreted.

Our previous modeling runs and the earlier

work done by Gunderson suggest that a wide

horizontal layer of insulation is effective at slowing frost penetration. We modeled this on the

Second Avenue pipe configuration by using a 6-

cm (2.4-in.)-thick layer of insulation in a total

width of 2 m (6.5 ft). The numerical simulations

predicted that this would prevent the 0°C isotherm from contacting the pipe. The problem,

however, is that this width of insulation is usually impractical to excavate for in a normal utility

line installation. That is why it is often preferable

to use the inverted U configuration shown in Figure 5.

After modeling several different combinations

of insulation thickness and shield width, using

the inverted U, we found that a 10.2-cm (4-in.)-

thick shield with a total minimum width of 101.6

cm (40 in.) would prevent the freezing front from

touching the pipe.

We then modeled the pipe as being 1.37 m (4.5

ft) deep and found that there was not a practical

insulation thickness that could prevent the freezing front from touching the pipe. The next approach we took was to see if water flowing in the

pipe would supply enough heat to prevent freezing. We used a 15.2-cm (6-in.)-thick shield, 1.22-m

(4-ft)-wide, and ran the model with a pipe boundary temperature to simulate water flowing in it.

We then stopped the flow to see if the 0°C isotherm would progress to the pipe. In our model

this is a two-step procedure.

The first step of the numerical simulation is

performed with a specified pipe boundary temperature derived from the known water temperatures. This simulates the actual temperatures at

any time we might need. These are then used as

starting temperatures in the second step, for a

numerical simulation without the specified pipe

boundary temperatures. As noted earlier, this is

equivalent to a situation where no water is flowing in the pipe. We then run the model for any

length of time needed to determine if the 0°C

isotherm progresses to the pipe.

The protection of the water pipe in the above

situation is dependent upon the initial temperature of the water, the time of year the water stops

flowing, and how long it remains off. In our simulations, we showed the time of maximum frost

penetration to be approximately 85 days after 1

January, or 26 March, at about 2.5 m (8.2 ft) deep.

If the water flow stopped at this time, our model

showed it to be safe from freezing. We then

stopped the water at the end of February and ran

the model for 70 days or into the first of May. In

this scenario the pipe got very cold but did remain (barely) above freezing.

From the above numerical simulations, we felt

that a 1.2-m (4-ft)-wide shield with either 10.1- or

15.2-cm (4- or 6-in.)-thick insulation, depending

upon how deep the pipe was buried, would be

adequate.

LABOSSIERE STREET

Initial design

When the initial excavation began on Second

Avenue it was discovered that the existing pipe

leading into the avenue was tightly surrounded

by ledge. It would have been very difficult and

expensive to excavate around it while still maintaining water service to the dwellings on the line.

We decided to move to our second-choice street—

Labossiere. The water line in this street was a

dead-end line about 123 m (405 ft) long, and

records showed that the existing pipe was buried

from 1.2 to 1.4 m (4 to 4.5 ft) deep. We dug three

test pits along the length of the pipe, and these

pits indicated that the closest ledge was about

1.37 to 1.52 m (4.5 to 5 ft) below the surface. Since

this was essentially the same situation we had

thought we had at Second Avenue, we felt that

the design could remain the same.

Final design

It was quickly discovered during the initial

excavation for the new pipeline that ledge was in

fact present up to the surface for nearly the entire

length of the pipeline. In an incredible stroke of

bad luck, the only three places that showed ledge

down to 1.4 to 1.5 m (4.5 to 5 ft) were the three

spots where we had dug our test pits. Consequently, we found ourselves in a situation where

the pipeline was being installed and our design

was suspect. Again, this is because ledge has a

much higher thermal conductivity and contains

less moisture than soil. These two conditions allow the freezing front to advance faster and deeper

through ledge than through relatively moist soil.

A series of numerical simulations were made

that modeled ledge to the surface with a pipeline

buried 1.5 m (5 ft) deep. This depth was close to

6
where the pipeline initially started out at the beginning of the street. During the blasting and excavation we tried to maintain at least this depth

to give us an extra buffer against the cold surface

temperatures.

The numerical model predicted that the pipe

would get very close to 0°C but would not go

below it by using our final design from Second

Avenue: a 1.5-cm (6-in.)-thick extruded polystyrene insulation with a total shield width of 1.2 m

(4 ft) with the pipe resting at the 1.5-m (5-ft)-deep

level.

Several points should be made about our designs based upon the numerical simulations:

1. The temperatures used at the model surface

are air temperatures from a very cold year. The

temperatures used at the model surface should

be ground-surface temperatures, which tend to

be warmer overall than the air temperatures because of the effect of solar energy. This study will

help to quantify this effect.

2. The ledge is modeled as if it were solid unbroken rock. Natural ledge is frequently broken

and can have areas of water flow and other anomalies. We expect the effective thermal conductivity

of the natural ledge to be less than our modeled

ledge.

3. Our failure criteria could be much too conservative. We “turn off” the water flow and assume that the pipe fails if afterwards the 0°C isotherm touches it. In reality there is probably some

water flowing frequently within the pipe, which

would bring a little heat into the shield area.

4. A great uncertainty exists in the water temperature boundary that we applied to the pipe.

As mentioned above, we used the water temperatures at the treatment and filter plants for our

pipe boundary temperatures. This may or may

not be correct, depending upon how the water

temperature changes as it travels through the distribution system.

5. Another concern in this study is the effect

that Berlin’s very cold water temperatures will

have within an insulated system. Will the cold

temperatures quickly remove the stored heat

within the insulation shield, much faster than

would have occurred had the heat loss only

been caused by heat conduction to the surrounding ground? One of the ways an insulated system is effective is by preventing the

rapid loss of this stored energy to the surrounding ground. The cold pipe temperatures could cancel out this benefit.

CONSTRUCTION

General physical layout

Labossiere Street is a dead-end hillside street

on the northwest side of Berlin. Our test section

started at the intersection of Sixth Avenue and

continued up the roughly 17% slope of Labossiere

to its end at the base of a mountain of bare rock

that projects upwards from the end of the street.

The water line serving the users on the street is

a dead-end line that runs about 128 m (420 ft) up

the west side of the street.

Pipe construction

The shield design called for a 15.2-cm (6-in.)-

thick layer of insulation in an inverted U around

the pipe. The sides of the U were 61 cm (2 ft) high,

with the bottom of the legs even with or slightly

below the bottom of the pipe. The total width of

the shield was 1.2 m (4 ft). The shield was constructed of 5.1-cm (2-in.)-thick, 4- × 8-ft SSE boards

of Foamular® 250 extruded polystyrene. This

material meets the specification requirements

of ASTM C578, Type IV. The boards have lines

scored at 40.6, 61, and 81.3 cm (16-in., 24-in. and

32-in.) spacing across their width, which make

them easy to break at the job site and leaves a

clean even line where they break. Consequently,

the 61-cm (2-ft)-high sides of the shield could

easily be obtained from the 4- × 8-ft. (1.2- × 2.4-m)

boards. Table 2 lists insulation data.

The new 8-in. ductile iron pipe construction

started with a T connection into the existing water line at Sixth Avenue and proceeded up

Labossiere Street. We did not start insulating the

line from this T because of the presence of a storm

drain line that ran down Sixth Avenue and crossed

over the new water line just up from this connection, as shown in Figure 6. We were concerned

that cold air would be flowing through the

underdrain and would cool down the water pipe

where they cross, so we installed a thermocouple

to monitor the temperature at the bottom of the

drain pipe. The stations shown in Figure 7 start

where the storm drain crosses the water line. The

Table 2. Insulation data.

Thermal conductivity Density Specific heat

(J/hr m °C) (kg/m3) (J/kg °C)

Foamular® 250 93.46 28.84 1339.8

7
insulation shield begins about 2.4 m (8 ft) past the

underdrain.

The top of the new water line at the storm

drain is about 1.5 m (5 ft) below the surface. A

backhoe was used initially to excavate for the

new line, as the ledge that was encountered was

relatively soft. We quickly ran into very competent ledge, however, and the ditch line had to be

blasted; a larger excavator was brought in to remove the debris. We tried to clear the rock down

to 1.8 m (6 ft) deep with a minimum 1.2-m (4-ft)-

wide ditch. The width varied depending upon

the amount of rock that was loosened in the blasting.

After the loose rock was removed, a layer of

sand about 15.2 cm (6 in.) deep was added to

cushion the pipe on the ledge at the bottom of the

ditch. We positioned some insulation jigs made

from 2- × 4-in.-diam. lumber into this sand and

along the centerline of the pipe layout to hold the

bottom of the side pieces of insulation. These jigs

prevented the sides from springing out at the

bottom during the backfilling operation. At the

top we used similar jigs to firmly hold the top of

the sides from moving in or out during the backfilling. These top jigs also maintained a 1.2-m (4-

ft) width at the top of the sides. Figure 8 shows

both the bottom and top jigs.

A section of pipe was laid on top of these jigs

and centered in the ditch, after which the sides of

the shield were installed. It is important to provide even mating surfaces and to stagger the insulation joints to prevent a direct path for heat

loss out of the shield. Three layers of the 5.1-cm

(2-in.)-thick extruded polystyrene insulation were

necessary to get our final design thickness, so the

boards were staggered as shown in Figure 9. Any

slope changes were made gradually to avoid a

gross vertical misalignment of the side boards.

The top jigs were added after the sides were installed, and together with the bottom jigs they

held the boards firmly in place during the backfilling. The sand backfill was alternated between the

inside and outside of the shield sides to prevent

the walls from caving in under the soil pressure.

It was brought up even with the top of the side

boards, the top jigs were removed, and a vibratory compactor was used to compact the sand. At

this point the sand inside the shield was evened

off, usually by dragging the back of one of the jigs

across the tops of the side insulation. This made a

smooth, level surface for the top insulation to rest

on. The top insulation was staggered in the same

manner as the sides and laid down over the prepared surface above the shield sides. Care should

be taken to line up the edges with the sides and to

8

Figure 6. Storm drain location, immersion thermocouple, and the start of the insulated pipe section on

Labossiere Street.

Figure 7. Instrumentation layout along

the pipeline.

0+19.6 Flow Meter

0+15.0 Storm Drain

0+09.9 TC1 (Immersion TC)

0+00

2+46 Pipe Cable, Inside Center Cable, Side

 Cable, and Outside Top Center Cable

 Thermocouples

2+39 Immersion TC

2+43 Offset 18 ft West, Control TC

2+31 Sewer Pipe Crossing

4+17 End of Pipe

4+10 Immersion TC

Pipe Centerline

N
avoid any gaps that would provide a path for

heat loss.

Figure 10 shows a service tap entering the

shield. These taps were insulated with closed-cell

pipe insulation, and the hole made in the insulation shield was foamed with a closed-cell spray

insulation. Figure 11 shows a residential sewer

line crossing through the insulation shield at approximately 70 m (231 ft) up the pipeline. The

sides of the shield were notched so the insulation

would fit over the pipe, and then insulation was

fitted into these notches under the water pipe

after the sides were installed. Some spray foam

was added to close up any remaining gap be4 ft

6” 6”

Top

4 ft

Bottom

Figure 8. Jigs made from 2- × 4-in. lumber to

hold the insulation during installation.

Figure 9. Staggered insulation

board joints.

Figure 10. Insulated service tap entering the shield.

9
tween the sewer pipe and the insulation, as shown

in Figure 11.

The shield was constructed in the manner described above up to the end of the line at approximately 127 m (417 ft). It would have been desirable to run the insulation shield 1.5 to 3 m (5 to 10

ft) past the end of the pipe, but ledge prevented

this. The installation of the shield was not the

limiting factor in the speed of the pipe laying; the

ledge blasting and removal took considerably

longer than the time needed to install the insulation. Backfilling was slowed slightly by the extra

care taken around the insulation sides, but this

time was minimized after we gained confidence

in the ability of the jigs to keep the insulation

from moving during the process.

INSTRUMENTATION LAYOUT

The general experimental plan called for monitoring temperatures at several points along the

pipe with thermocouples, detailed below, as well

as an extensive thermocouple layout at a test section about 73 m (240 ft) up the pipeline. In this

application we used copper/constantan, type T

thermocouples. The temperature data is automatically recorded by a Fluke 2286 datalogger that

was installed in the basement of a house on

Labossiere Street. This house is about 110 m (360

ft) up the street and about 37 m (120 ft) from our

main test section. We also installed water flow

meters in the pipe at two locations but were disappointed to discover that both failed to function

properly. Figure 7 is a profile view of the as-built

pipeline with several points of interest marked.

Three immersion thermocouples were installed

into the pipe at 3.1, 73, and 125 m (10, 240, and

410 ft) from the beginning of the insulation. These

allow us to monitor the water temperature as it

progresses up the street. At our main test site we

have also installed thermocouples around the outside of the pipe near the middle immersion thermocouple. This will allow us to tell if there is any

significant difference between the temperatures

of the water and the outside of the pipe.

The bottom immersion thermocouple gives us

the temperature of the water near the entrance of

the test pipe. This will be a baseline temperature

of the water as it enters the test section, before

any influence of the shield. A flow meter was

installed at 6.1 m (20 ft) up the pipe but, as mentioned before, it is not working properly. We had

hoped to know the water flow into the test section by this meter. Another thermocouple was

placed on the outside of the pipe at about 24.4 m

Figure 11. Residential sewer line crossing through the shield.

10
(80 ft). The sewer line crossing at 70.4 m (231 ft)

gave us an opportunity to measure the effect, if

any, that the relatively warmer sewer line would

have on the temperatures within the shield. Figure 12 shows the sewer line and the placement of

the four thermocouples at this location.

The main instrumentation section was at approximately 73 m (240 ft) along the pipe. Here we

installed 40 thermocouples, as shown in the profile in Figure 13. We also installed an immersion

thermocouple and a flow meter at this location.

As before, this flow meter does not appear to be

working. The thermocouple layout at the main

test section will enable us to characterize the thermal environment around the shield. The control

string will give us temperatures that we would

expect to find in an undisturbed area. The temperatures obtained at the surface of the test section will be used as our surface boundary temperatures in the numerical model, and the

temperatures from the immersion thermocouple

will be used as our pipe boundary temperatures.

Three more thermocouples were installed at

the site. One is on the normally dry portion of a

hydrant standpipe just above the main test section at about 104 m (340 ft). There is an immersion thermocouple at the end of the pipeline at

125 m (410 ft) and an outside air temperature

thermocouple off the outside north wall at 73

Labossiere Street. We planned to compare air and

surface temperatures using this thermocouple and

the previously mentioned surface thermocouples

at the main test section.

PRELIMINARY DATA

Air temperatures

and frost penetration

The winter of 1994–95 was unusually warm and was therefore not an

optimum test for the shielding concept. Some of the temperatures collected from the beginning of data collection on 9 September 1994 until 17

March 1995 are shown in Figure 14.

Here the outside air temperature is

compared with the numerical surface boundary temperature used in

the design model. As mentioned previously, this FE boundary temperature was determined from air temperature records in the Berlin area. It

can be seen that the numerical temperatures were generally colder than the air temperatures. Probably just as important is that when

we did experience cold temperatures, they did

not last very long. Notice that the temperatures

above 0°C tend to last longer than those below

0°C, and there is never a prolonged cold spell.

Under this type of temperature profile the ground

never has a chance to maintain any appreciable

frost penetration. In our situation in ledge, the

Figure 12. Sewer line crossing with thermocouple

layout.

11

W E

87.31 ft

87.98 ft

88.57 ft

Sewer Pipe

1

2

3

4

Sewer Pipe

1

2

3

4

5

6

7

8

9

10

11

12

16

36

35

34

33

29

27

25

23

22

21

19

18

17

Control

String

–6.75 ft

42

39 41

38

37

14

15

40

24

28

26

20

–4.96 ft

–7.05 ft

–7.63 ft

–9.05 ft

–3.43 ft

–6.48 ft

PVT

–5.0 ft

–1.81 ft

32

Figure 13. Thermocouple layout at the test site.

–7.05 ft
frost penetration reacts quickly to surface

temperature conditions. Figure 15 shows

how rapidly the 0°C isotherm changed

depth in the ledge, depending upon the surface temperatures. The maximum frost penetration that we recorded on Labossiere

Street was about 1.1 m (3.5 ft).

Pipe temperatures

Berlin has very cold water in its distribution system, which makes it a challenge to

design effective insulation. Figure 16 is a

graph of water temperatures at the treatment plant and at the immersion thermocouples in the pipe at Labossiere Street. It is

evident from this figure that in September

the water at the plant is warmer than at the

beginning of Labossiere Street (the lower

immersion thermocouple) and has

therefore lost heat to the ground as it

flowed to Labossiere. As the winter

season progresses, there are periods

where the treatment plant water has

warmed up as it has traveled to

Labossiere.

The lower and middle immersion

thermocouples are roughly the same

depth below the surface, and the upper thermocouple is 1.2 to 1.5 m (4 to

5 ft) deeper. We can compare the lower and middle thermocouples (Fig.

17) and see that the middle thermocouple is about 0.5°C warmer than

the lower, showing the temperature

rise of the water as it travels up the

street. The upper thermocouple shows the

warmest temperatures, but this would be expected because of its greater depth. This makes

it difficult to use the upper thermocouple to

draw any conclusions about any beneficial

effect from the insulation.

We were concerned that cold air flowing

through the storm drain that passes over the

water pipe at the beginning of Labossiere would

hasten freezing in the immediate area. As can

be seen from Figure 18, the temperatures

recorded at the bottom of the storm drain were

in fact warmer than the lower immersion thermocouple nearby. We do not know if this would

hold in a year with colder air temperatures.

Shield temperatures

In comparing temperatures inside and outside the shield at the 1.5-m (5-ft) depth (Fig.

–20

–10

0

10

20

30

–30

 3 Sep 13 Oct 22 Nov 1 Jan 10 Feb 22 Mar

 Temperature (°C)

 Air

 FE Surface

1994 1995

Figure 14. Air temperature vs. FE surface temperature.

0 –1 –2 –3 –4

Depth (ft)

2

1

0

–1

–2

Temperature (°C)

Control 8 Dec 94

27 Mar 95

17 Mar 95

6 Mar 95

22 Dec 94

30 Jan 95 12 Jan 95

13 Feb 95

Figure 15. Control string vertical temperature profiles.

Figure 16. Comparison of lower and middle immersion

thermocouples and filter plant temperatures.

3 Sep

16

12

8

4

0

23 Oct 12 Dec 31 Jan 22 Mar

Temperature (°C)

Low Imm (–4.76 ft)

Mid Imm (–5.26 ft)

Treatment Plant

1994 1995

12
19), it is evident that the inside temperature is

colder than the adjacent outside temperature. This

is a result of the cold water temperatures in the

shield and the fact that it was a warm year. In a

normal year we would expect the inside shield

temperature eventually to become warmer than

the outside when the colder temperatures penetrate deeper into the ground as the year progresses. Figure 19 also shows the control temperatures, which are in ledge, becoming colder

than the equivalent depth at the pipe, which is in

sand backfill. It is evident in Figure 20 that the

ground temperature at the depth of the shield did

not get colder than the water temperatures.

NUMERICAL MODEL

ASSESSMENT

The first shield was designed using air temperatures that we hoped would be representative

of an extremely cold year. This was a conservative approach both from the climatological standpoint as well as the numerical modeling standpoint because the numerical model uses surface

temperature, which is usually warmer than air

temperature. Figure 21 shows the actual surface

temperature from our control string on Labossiere

and two regression lines. One regression represents the FE boundary temperature used in our

0

2

4

6

8

10

12

14

16

18

Temperature (°C)

3 Sep 13 Oct 22 Nov 1 Jan 10 Feb 22 Mar

Low Imm (–4.76 ft)

Underdrain (–4.07 ft)

Ground (–4.00 ft)

1994 1995

0

4

8

12

16

20

Temperature (°C)

3 Sep 13 Oct 22 Nov 1 Jan 10 Feb 22 Mar

FE Outside (–5 ft)

Actual Outside (–5 ft)

FE Inside (–5 ft)

Actual Inside (–5 ft)

Control (–5 ft)

1994 1995

3 Sep

18

16

14 

12

10

8

6

4

2

0

13 Oct 22 Nov 1 Jan 10 Feb 22 Mar

Temperature (°C)

Ground (–5 ft)

Actual Water Temperature

FE Pipe Temperature

1994 1995

Figure 17. Immersion temperature vs. ground temperature.

Figure 18. Comparison of lower immersion thermocouple,

underdrain, and earth temperatures.

Figure 19. Inside vs. outside shield temperatures.

Figure 20. Pipe vs. ground temperature.

10

 8

 6

 4

 2

 0

2 Nov 22 Nov 12 Dec 1 Jan 21 Jan 10 Feb 2 Mar 22 Mar

Temperatrue (°C)

Upp Imm (–9.82 ft)

Mid Imm (–5.26 ft)

Low Imm (–4.76 ft)

Ground (–4.81 ft)

1994 1995

13
shield design, and the other is one that fits the

actual surface temperature. When the two regressions are compared it shows that the actual surface temperature was, on average, over 5.5°C

warmer than the numerical surface temperature.

To better determine how the numerical model

performed, the top and pipe boundary conditions

were changed to reflect the actual temperatures

more closely. To do this, we need a year’s surface

and pipe temperatures covering the model year

of interest. We had been gathering data on Second Avenue for over a year, and these surface

temperatures were compared with the available

Labossiere temperatures and adjusted to represent the Labossiere temperatures from March 1994

to March 1995. The treatment plant water temperatures from the same time period were

used as our pipe temperatures.

Figures 19 and 20 also show the results

from our numerical model. In Figure 19, the

numerical model overpredicts the inside

shield temperatures from September until

about the first of December, but it does very

well until the end of data in March. The

overprediction is in part because our numerical pipe temperatures (from the treatment plant) are warmer than the actual pipe

temperatures during September to December. The outside-the-shield numerical temperatures are at first warmer than the actual, until about mid-November. From this

point until the end of December they represent the actual temperatures well. From then

until the end of data in March our numerical model is somewhat colder than the actual temperatures.

The numerical model did poorly on predicting the overall frost penetration as compared with the control string temperatures

in ledge. The maximum frost penetration

predicted by our numerical model was

roughly 1.8 m (5.9 ft) in about the mid to

end of February. The actual maximum frost

penetration was about 1.1 m (3.5 ft) during

the first part of March.

The colder numerical temperatures and

deeper modeled frost penetration could be

due to our ledge thermal conductivity and/or

the nature of the regression model and how

it represents the actual temperatures.

Figure 22 shows a portion of the control

surface temperatures from the beginning of

November until the beginning of March.

Notice that once the regressed surface temperature line goes below 0°C it remains there until spring. In a normally cold year the actual temperatures are usually modeled well by this type

of regression. However, in the winter of 1994–95,

the actual temperatures rose above this regressed

line and above freezing for a significant amount

of time, as shown in the figure.

To check the effect that the regression line has

upon frost penetration, we used actual surface

temperatures rather than the regressed temperatures as the top boundary condition. The 0°C isotherm reacted much more realistically in this

scenario, with the ground temperatures rising

above 0°C during some of the warmer surface

temperature times. The maximum frost penetration time was still around the mid to end of

15

–15

10

–5

5

0

–10

FE Surface

Actual Surface

12 Nov 2 Dec 22 Dec 11 Jan 31 Jan 20 Feb 12 Mar

Temperature (°C)

1994 1995

Figure 21. FE surface temperature vs. actual surface temperatures, Laboissiere Street.

Figure 22. FE surface vs. actual temperatures.

35

25

15

5

–5

–15

–25 Temperature °(C)

3 Sep 3 Oct 2 Nov 2 Dec 1 Jan 31 Jan 2 Mar 1 Apr

Surface (94-95)

FE Regression

Regression of Actual

Surface Temperature

1994 1995

14
February and decreased to 1.7 m (5.6 ft), a small

improvement.

As mentioned earlier, we model ledge as a continuous mass of solid rock where actual ledge

might not be so. During the construction of the

pipeline it was evident that portions of the ledge

were fractured, with seams of water within some

of the fractures. These areas would greatly influence the thermal conductivity of the area and

would slow frost penetration compared to solid

rock. The above analysis suggests that we have a

higher numerical ledge thermal conductivity

than is appropriate.

PRELIMINARY CONCLUSIONS

AND RECOMMENDATIONS

The winter of 1994–95 was too warm to provide us with a good test of the shielding concept.

We still do not know what the water temperatures will be at the test site during a very cold

year or what effect they will have upon the shield’s

performance.

It appeared that the shield was providing some

protection to the water line, as the temperature of

the water increased as it flowed up the hill. It is

difficult to draw any firm conclusions, however,

since the ground at the pipe depth remained

above freezing throughout the winter.

Another area of concern is the performance

of the service lines to the residences. These

small-diameter lines are relatively shallow and

uninsulated and would seem to be susceptible

to freezing.

The numerical model appears to be functioning well where we have accurate input to check it

against. In our future comparisons we will be

using the actual surface temperature as our top

boundary condition to see if we can more closely

model the maximum frost penetration depth. The

results of these runs will help tell us if the ledge

thermal conductivity needs to be adjusted and

what effect inaccuracies in the material thermal

conductivities have upon our model results.

LITERATURE CITED

Coutermarsh, B.A. and G.E. Phetteplace (1991a)

Analysis of frost shields using the finite element

method. In Proceedings of the Seventh International

Conference, Numerical Methods in Thermal Problems

(R.W. Lewis, J.H. Chin and G.M. Homsy, Ed.),

Vol. VII, Part 1, p. 122.

Coutermarsh, B.A. and G.E. Phetteplace (1991b)

Numerical analysis of frost shields. Cold Regions

6th International Specialty Conference (D.S. Sodhi,

Ed.), American Society of Civil Engineers, p. 178.

Gunderson, P. (1975) Frostproofing of pipes. U.S.

Army Cold Regions Research and Engineering

Laboratory, Draft Translation 497.

Gunderson, P. (1989) Distribution network for

district heat, electricity, water and sewage. Norwegian Building Research Institute, Oslo, Project

Report 46 (in Norwegian).

Lunardini, V.J. (1981) Heat Transfer in Cold Climates. New York: Van Nostrand Reinhold Company.

Phukan, A. (1985) Frozen Ground Engineering.

Englewood Cliffs, New Jersey: Prentice Hall,

Inc.

Segerlind, L. (1984) Applied Finite Element Analysis. 2nd edition. New York: John Wiley and Sons.

15
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January 1997

Frost Shielding Protection of a Water Line, Berlin, New Hampshire

Barry A. Coutermarsh

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road Special Report 97-1

Hanover, New Hampshire 03755-1290

Office of the Chief of Engineers

Washington, D.C. 20314-1000

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International System of Units, published by the American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa.

19103.

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161

Frost penetration Insulating buried pipes Utility line freezing 25

Frost protection of pipes Pipe freezing Water line freezing

Frost shielding Sewer line freezing

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

The standard practice of burying water and sewer lines beneath the frost line in cold regions can be expensive when

ledge or other difficult material is within the burial depth. If the pipeline can be buried at a shallower depth and

still be protected from freezing, a significant savings in excavation costs can be realized. A finite element (FE)

program was developed to predict frost penetration depth around buried utility pipelines. The program was used

to design and assess the feasibility of various insulation configurations around a water line buried within the frostsusceptible depth in Berlin, New Hampshire. Extensive temperature monitoring was performed to evaluate both

the insulation design and the prediction accuracy of the FE program. The first-year results are very promising,

showing good accuracy between the FE results and actual temperatures.